The present invention relates to a method for performing measurements using a test element in a coordinate-measuring machine or machine tool, wherein the test element in the method is arranged in a different pose.
The accuracy requirements relating to the positioning of rotation axes, for example rotatable joints or rotating apparatuses, which are used in coordinate-measuring machines (CMM) or machine tools, frequently exceed the accuracies of the measuring systems of the rotation axes, for example of an angle-measuring system. It is therefore customary to capture the error of the measuring system or of a scale which is contained therein in a suitable fashion and to correct it downstream, for example during the measurement operation.
One possible variant for capturing a position error of a rotation axis is capturing the error using a mirror polygon in which angles between the mirror surfaces are exactly known. A mirror polygon of this type is an arrangement of mirror surfaces which are arranged around a common point of rotation. Known are for example polygons with 8 surfaces and up to 36 surfaces. The angles between the mirror surfaces are exactly known, since they were ascertained with high precision by certified organizations.
If a mentioned polygon is placed on a rotation axis for which the error of the angle-measuring system is intended to be ascertained, the angles of the polygon can be measured for example in an autocollimation telescope or another suitable angle-measuring machine. The deviations between predetermined angles of the calibration certificate and the angles indicated by the angle-measuring system are in that case errors of the angle-measuring system or of its scale itself.
However, this method has the following disadvantages:                Limited calibration accuracy. The accuracy of the ascertainment of the error of the angle-measuring system increases as the number of mirror surfaces increases. The greater the number of mirror surfaces, the more accurately the calibration can be performed. However, polygons having more than 36 surfaces are not practical anymore.        A calibrated polygon is difficult to procure, because generally long delivery times must be expected.        High purchase costs for a calibrated polygon.        The mirror surfaces are very sensitive to external influences, which makes handling difficult.        
The application WO 2014/108187 A1 proposes to perform the calibration of a rotation axis to be tested, for example of a rotary table, against a calibrated rotating apparatus. In the process, both rotating apparatuses are stacked one on top of the other and rotated in opposite directions. A mirror is placed on the stack and used to capture the difference in rotation of both rotation axes using an autocollimation telescope and to thus ascertain the rotation error of the rotating apparatus to be tested. This method offers the advantage of freely selectable support locations. However, first the reference rotating apparatus needs to have been exactly calibrated. The disadvantage of this is that any residual errors in the reference rotating apparatus are not also taken into account but are interpreted as errors of the rotating apparatus that is to be tested. Furthermore, highly accurate calibration of the reference rotating apparatus is also a challenge which cannot be overcome with trivial means.
To solve the just mentioned problem, WO 2014/108187 A1 proposes to perform what is known as a reversal measurement. Instead of measuring the difference between angle measurement values between the reference rotating apparatus and the rotating apparatus that is to be tested with only one mirror, for example two mirrors, arranged opposite each other, are attached to the rotating apparatus to be tested and measured one after the other. A method of this type is described in detail in WO 2014/108187 A1 with reference to FIGS. 24a to 24d, 25 and 26. The disadvantage of this method is that other error components, such as a dual waviness of the error of the reference rotating apparatus, are not taken into consideration. For capturing this residual error of the reference rotating apparatus, four mirrors must be present, which requires a correspondingly greater outlay. To this end, that is to say for providing four or more mirror surfaces, a polygon could in turn be used. In the reversal measurement, a further rotation of the polygon can be effected via two rotating apparatuses which are stacked one on top of the other and rotate in opposite directions during the measurement. An exactly calibrated polygon in which the angles between the mirror surfaces are exactly known is not necessary herefor, because the polygon angles can be determined in the course of such a measurement and to do not need to be known in advance. However, other disadvantages of the use of a polygon remain, primarily the disadvantage of the limited number of support locations which is determined by the number of the mirror surfaces of the polygon. What is disadvantageous in such a reversal measurement using a polygon is that correction of errors of the reference rotating apparatus of an even higher order is not possible. For example, it is not possible to correct an error of a higher order, for example 30th ripple, using a polygon having 36 surfaces. The reason herefor is that the number of reversal measurements is determined by the number of mirror surfaces of the polygon. The number of possible reversal measurements is a result of the integer divisor of the mirror surfaces, so in the case of 36 surfaces: 2, 3, 4, 6, 9, 18, 36. Consequently, for each special case, such as a reversal measurement with a specific number of desired reversals, a dedicated polygon would have to be provided, which in terms of cost and outlay would simply not be acceptable. In addition, due to the fact that the reflective surface of the polygon must not be smaller than a specific size, the implementability of a polygon having more than 36 surfaces is not possible without increasing the dimensions of the polygon itself, but this would result in a polygon which is too large and/or too heavy.
It is the object of the invention to specify a solution for one or more of the above-stated problems.